Successive log video pad power detector and method

ABSTRACT

A power detector which samples the output signal from a communications device and produces a control signal proportional to the transmit power level, comprising a series of diode detectors where the sampled signal is divided between the diode detectors by an attenuator cascade.

BACKGROUND OF INVENTION

[0001] Applicant's disclosure is directed to an improved method andsystem for controlling the transmit power level in a communicationdevice. Specifically, applicant's disclosure is directed to a powerdetector having an improved dynamic power range that is less complex andcheaper to construct than conventional power detectors having a similarpower range.

[0002] The use of signal amplifiers in radio communications is wellknown. For hand held communication devices, federal regulations,collision avoidance and battery power conservation are factors whichdictate tight control over the transmitted power level of the RFsignals, the goal being to transmit at sufficient power to ensurereliable reception of the transmitted signal while avoiding unnecessarydrain on battery resources and avoiding signal collision with othercommunication devices.

[0003] A conventional method for determining transmit signal power levelis to use a simple single stage diode envelope detector. FIG. 1 is asimplified diagram of one such prior art power detector. An RF inputsignal is received by the power control circuit 10 where the signal isamplified or attenuated as necessary and then sent to antenna 12 wherethe RF output signal is transmitted. Prior to transmission, the RFoutput signal is sampled by a coupler 14. The coupler 14 provides thesampled signal to the diode detector 16. Detector 16 generates a signal(normally DC volts) which is proportional to the output power of the RFsuit signal. The detector signal is then compared at comparator 18against a reference signal which corresponds to the desired output powerlevel of the transmitter. The output of the comparator 18 drives theamplification/attenuator network 10 which either increases or decreasesthe power level as necessary to control the transmit power level.

[0004] The single diode detector is simple and cheap method of powercontrol in communication devices. However, there are several limitationsto the diode detector which may effect product performance and add coststo the power detector.

[0005] The simple diode detector has a narrow practical operating powerrange. The nonlinear response of the diode detector is a square lawfunction, not a log-linear detector. Though a square law curve is not abad approximation for a log-linear curve over a small operating range,wide dynamic ranges present the need for a level dependent correctionfactor. For example and with Reference to FIG. 2, at high powers 1.000volt might represent a 5 dB change in power, where at low powers 0.010volts might represent the same 5 dB change in power.

[0006] Thus, for the lower signal levels sensitivities of 2 mV/dB arenot uncommon. To maintain a 10 dB signal to noise ratio the noise floormust be below 2001 mV—a level difficult to achieve in a noisyenvironment.

[0007] Additionally, a 10 to 15 dB dynamic range is all that is useable,because the detector's non-linear response rapidly drops the detectedoutput voltage for low power input signals making noise immunity anissue.

[0008] Another important concern is that the repeatability of the signaldiode detector curve is not very good, which is critical if multiplediodes are used together. The curves are a strong function of the dopinggradients in the semiconductor, the load presented to the diode and thetemperature of operation. It is possible to control the load fairlyconsistently, however the doping gradients vary as a function of manyprocessing steps, which are difficult to control. Even if diodes werematched and loads were controlled carefully, the curves varysignificantly with temperature.

[0009] Changes in ambient temperature in the environment of the diodecan significantly change the output of the diode. The temperature effecton a diode detector are quite pronounced and attributable primarily tochanges in the forward barrier and video resistance. Unfortunately,neither property is easily controlled and thus it is usually necessaryto use some sort of temperature compensation circuit to remove theeffects of temperature from the output of the diode detector.

[0010] In summary a single diode detector only has a narrow power rangewhere repeatable, stable, good signal to noise measurements can be made.For wide dynamic ranges (40 dB) another solution must be sought.

[0011] The Successive Log Video Amplifier (SLVA) concept has been aroundfor many years and provides an accurate log-linear response over verywide dynamic ranges. The SLVA utilizes a series of diode detectors, witheach detector operating in a narrow power range. The detectors onlyreceive a portion of the sampled signal and the output of the detectorsmust be summed to provide an accurate representation of the transmitsignal level. The use of a series of diode detectors requires that thesampled signal be divided among the detector. The SLVA uses a amplifiercascade to provide portions of the sampled signal to the detectors.

[0012] The SLVA is built from a successive cascade of very carefullygain-matched amplifiers. Between each amplifier a simple diode detectoris placed. Each amplifier typically has a gain of about 10 dB. Thus, thediode detectors only need to detect power over the range where thedetector is well behaved and has good noise immunity. The outputs of thedetectors are then summed together in such a way to produce a closeapproximation to a log-linear response. SLVA's are commonly availablewith ten plus stages with accuracy being better than 1 dB over a 100 dBdynamic range.

[0013] With the exception of a few monolithic designs, SLVA's arehowever typically very expensive. The reason for the high cost is theneed for very accurate gain matching. This matching requires highskilled labor and significant tuning time. Higher frequency applicationsrequire closer matching thus increasing the costs even further. Forexample, at microwave frequencies, each stage must be gain matched tooften better than 1 dB.

[0014] Thus, the concept of using a series of amplifiers with detectorsoperating over a narrow range and summing the detectors works well forlow power signals and thus the SLVA is almost always used for detectingvery low power signal levels. This device is commonly used in both radarsystems and spectrum analyzers and recently this technology has beenapplied via a monolithic device from Analog Devices. However, suchmonolithic devices are not available above 3 GHz or above +10 dB andalthough not as expensive as traditional gain matched amplifier cascade,monolithic devices still require significant development costs.

[0015] Recently communication devices operating in the very highfrequency (VHF) spectrum have been made available for commercial use,e.g. milimeter wave systems. Such high frequency systems are susceptibleto significant signal degradation due to atmospheric conditions. Forexamples, very high frequency signals suffer significant attenuationduring rain storms. One method of mitigating this rain attenuationeffect is to increase the power level of the transmitted signal in orderto “burn” through the signal to the receiving station. When the rainattenuation effect is not present the power level of the transmittedsignal can be reduced and thus battery power of the communication deviceis conserved. Thus, there is a need for a power detector which worksover a broad power range, i.e. both high and low power, without theattendant costs and time associated with matched gain filters.

[0016] Accordingly, it is an object of the present invention to providea novel power detector which has a dynamic power range and works at bothhigh and low power.

[0017] It is another object of the present invention to provide a novelpower detector which can be used at high frequency without the attendanttime and costs associated with gain matched filters.

[0018] It is yet another objective of the present invention to provide anovel power detector which utilizes low cost reliable diode detectorswithout an associated amplifier cascade.

[0019] It is still another object of the present invention to provide anovel power detector for use with high frequencies at high and lowpowers without the use of cascaded active elements.

[0020] It is yet still another object of the present invention toprovide a novel power detector which reduces the costs and time requiredto manufacture.

[0021] It is still another object of the present invention to provide anovel method of generating a power control signal proportional to thetransmit power level for high and low power applications.

[0022] These and many other objects and advantages of the presentinvention will be readily apparent to one skilled in the art to whichthe invention pertains from a perusal of the claims, the appendeddrawings, and the following detailed description of the preferredembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a simplified block diagram of a single diode detector.

[0024]FIG. 2 is a typical operating curve for the single diode detectorof FIG. 1.

[0025]FIG. 3 is a simplified block diagram of the Successive Log VideoPad (SLVP) power detector utilized by the present invention.

[0026]FIG. 3 represents one embodiment of applicant's disclosure. Thedesign consists of five main building blocks, the pie pads 30, diodedetectors 32, high impedance buffers 34, saturating gain amplifiers 36and a summer 38.

THE DETAILED DESCRIPTION

[0027] A seven stage detector requires a little more than 5 dB dynamicrange from each detector stage. It also fits with in the bounds of 4quad opamp devices (leaving some spare capability for temperaturecompensation). Such detectors are stable over the entire 5 dB rangeproviding good signal to noise measurements. The detectors may be highsensitivity Zero Bias Diode (ZBD), mounted on top of the integratingcapacitor. Bias may not be applied for simplicity and thus the detectorstage can be very cheap and accurate.

[0028] The pad (attenuator) elements 30 may be discrete pie pads of 5dB, each bonded to the inside of a carrier module. The pads 30 may be amonolithic thin film part and thus would have the ratio-metricproperties of the mask. The shunt ground connection on the pad may beachieved via a wide trace and wire bond and assists in increasing thebandwidth of the device. The passive nature of the RF cascade in thepresent embodiment is unconditionally stable as compared with the activenature of the amplifier cascade in the prior art SLVA. Additionally, athin film pad is much smaller than the equivalent gain delta amplifier.

[0029] With respect to practical limitations, the realizable gain deltain applicant's disclosure can be very small relative to the practicalgain delta of a prior art video amplifier. For example, a 2 dB padcascade is easily realized with thin film technology for pennies, but a2 dB amplifier cascade, though do-able, is very impractical. Amplifiergain blocks tend to come in 8 to 14 dB steps, making additional paddingnecessary to achieve small sizes. Thus, the very fine, ultra log-linear2 dB steps are expensive to realize with amplifiers.

[0030] The ratio-metric precision of a thin film, thick film orsemiconductor resistor mask can produce near perfect log-linearresponses. This near identical gain delta between detectors along thepad cascade gives applicant's disclosure excellent performance withoutthe need for adjustment and without the need for gain matched amplifiersthat have limited bandwidth and power capability.

[0031] The broad band nature of the attenuators of applicant'sdisclosure provide unusually wide bandwidth capability. Currently priorart devices are available up to 3 GHz with ±11 dB non-tunable accuracyat low power. Applicant's disclosure is easily fabricated with standardthin film techniques to work at 40 GHz with the same nontuneableaccuracy. The use of cascaded passive elements allows for use with amuch broader frequency spectrum and the present embodiment works down toDC which conventional SLVA's can not do.

[0032] A buffer 34 may be placed after the detector to insure high DCinput impedance. This reduces the effect of the diode video resistanceand improves sensitivity.

[0033] A high gain operational amplifier (opamp) (A_(v)=400) 36 may beused to provide gain and the limiting function necessary for equal gainlog-linear summing. Opamps provide low input offset drift and saturationcharacteristics. Unlike the SLVA's which use the saturation of the videoamplifier, the present embodiment uses the saturation of op-amp 36 tolimit the contribution of each stage.

[0034] The independent nature of each high gain opamp 36 allowssuperposition to be used for summing, yielding a simple resistive summer38. Unlike the traditional SLVA, the present embodiment uses purelypassive summing of the detector 32 outputs. Because of the opampsaturation requirement to provide the truncation of the detector range,the opamp 36 is configured for high gain and high isolation. This allowsfor simple superposition of the op-amps' output sources to do thesumming.

[0035] The opamps 36 may be procured on a single monolithic substrate.The saturation of the opamps 36 is more predictable, abrupt andcontrollable than the RF saturation characteristics of the videoamplifiers typically used in the SLVA.

[0036] Typical log-linear errors for the present embodiment at 25° C.are less than 1.0 dB over all but the extremes of the power range. Athermistor compensation circuit (not shown) could implement simpletemperature linear correction using the two unused opamps.

[0037] In another embodiment of the applicant's disclosure, a singleamplifier added before the attenuator cascade allows the for accurateand cheap power detection at low powers. Because applicant's disclosurecan deliver excellent log-liner response over a 40 dB range withouttuning, it is less expensive to add a single gain stage before thecascaded pads to extend the detection range lower than to utilize agained matched video amplifier cascade as taught in the prior art.

[0038] Thus applicant's disclosure provides an easier, less costly,faster, broader band way to make a precision matched, temperature stableattenuator cascade for use to detect transmit power levels in high andlow frequency and high and low power applications than taught in theprior art.

[0039] While preferred embodiments of the present invention have beendescribed, it is to be understood that the embodiments described areillustrative only and the scope of the invention is to be defined solelyby the appended claims when accorded a full range of equivalence, manyvariations and modifications naturally occurring to those skilled in theart from a perusal hereof.

What is claimed is:
 1. In a power detector for a radio frequencywireless communications transmitter with a controllable transmittedpower level in which a sampled signal is provided via a plurality ofmatched cascaded elements to a plurality of diode detectors, the outputsignals from which are summed to provide a control signal for thetransmitted power level, the improvement wherein the cascaded elementsare passive.
 2. The power detector of claim 1 wherein said cascadedelements are attenuators.
 3. The power detector of claim 2 including anamplifier upstream of said cascaded attenuators.
 4. A power detector forcontrolling the transmit power level of a communications devicecomprising: plural cascaded attenuators for receiving a signalrepresentative of the transmit power level of a communication device andfor providing plural attenuator output signals; plural diode detectorseach receiving one of the plural attenuator output signals to therebyproviding plural detected signals; a summer responsive to the pluraldetected signals for producing a control signal having a voltageproportional to the power transmit power level of the communicationsdevice; and. a control unit for adjusting the transmit power levelresponsively to the control signal.
 5. The power detector of claim 4wherein said plural attenuators comprise a printed thin film successivepad cascade.
 6. The power detector of claim 4 wherein said pluralattenuators each have substantially the same ratio-metric properties. 7.The power detector of claim 4 further comprising plural buffersconnected one each between one of said plural detectors and said summerto thereby increase the d.c. input impedance and improve the sensitivityof the power detector.
 8. The power detector of claim 4 furthercomprising plural high gain operational amplifiers connected one eachbetween each of said buffers and said summer.
 9. The power detector ofclaim 8 wherein said plural operational amplifiers include portions of acommon monolithic substrate.
 10. The power detector of claim 4 furthercomprising an amplifier for the signal representative of the transmitpower level upstream of said plural attenuators.
 11. A power detectorfor controlling the transmit power level of a communications devicecomprising: means for providing a power level signal representative ofthe transmit power level of the communication device; passive means fordividing the power level signal into plural power level signals; meansfor detecting a characteristic of each of the plural power level signalsto thereby provide plural detected signals; and means for summing theplural detected signals to thereby provide a control signal.
 12. A powerlevel control circuit for a communications device comprising: acommunications device having a transmitted power level control; adetector for providing a power level signal representative of thetransmit power level of said communication device; a passive signaldivider for dividing the power level signal into plural power levelsignals; a unidirectional circuit for detecting a characteristic of eachof the plural power level signals to thereby provide plural detectedsignals; an adder for summing the plural detected signals to therebyprovide a control signal to said power level control.
 13. In a method ofdetecting the transmit power level of a communication device wherein asignal related to the signal transmitted by the communication device isdivided by a plurality of cascaded elements into plural components, andwherein a characteristic of the components is detected and summed toprovide a signal related to the power level of the communicationsdevice, the improvement wherein the division is accomplished using onlypassive circuit elements.
 14. A method of controlling the transmit powerlevel of a communication device comprising the steps of: a. providing apower level signal representative of the power level of the signaltransmitted by the communication device; b. attenuating the power levelsignal with a successive cascade of attenuators to thereby provide aplurality of attenuator signals; c. detecting a characteristic of eachof the plurality of attenuator signals to thereby provide pluraldetector signals; e. summing the detector signals to provide a controlsignal; and f. controlling the transmit power level of thecommunications device responsively to the control signal.
 15. The methodof claim 13 further comprising the step of amplifying the power levelsignal prior to dividing the power level signal.
 16. The method of claim13 further comprising the step of amplifying each of the detectorsignals prior to summing.
 17. In a method of controlling the power levelof a radio frequency transmitter wherein the level of power is detectedby sampling the transmitted signal, the sampled signal is divided, acharacteristic of each of the divided signals is detected by aunidirectional device, and the detected signals are summed to provide acontrol signal, the improvement wherein the sampled signal is dividedprior to detection without using active circuit elements.